Strategies are proposed to evaluate the potential of severe combined immunodeficient (SCID) mice to serve as an in vivo model to study mechanisms involved in homing of human lymphocytes. Lymphocyte migration into peripheral lymph nodes is initiated by the interaction of the L-selectin homing receptor on lymphocytes with the vascular addressin on post-capillary high endothelial venules (HEV). To determine if addressin expression develops normally in immunodeficient mice, a direct comparison will be performed of the level of the peripheral lymph node addressin expressed on HEV of normal Balb/c and SCID. The MECA-79 monoclonal antibody, which binds to both the murine and human peripheral lymph node vascular addressin, will be used to detect the presence of this antigen by immunohistochemical staining and Western blot analysis. Experiments are also proposed to determine if murine SCID peripheral lymph node HEV are functionally capable of mediating L-selectin-dependent adhesion of human lymphocytes. This aim will be accomplished by (a) directly comparing human lymphocyte binding in vitro to peripheral lymph node HEV of normal Balb/c and SCID mice and (b) examining the effect of L-selectin-specific blocking antibodies on the adhesion of human lymphocytes to murine HEV. Non-blocking L-selectin-specific monoclonal antibodies will be used as a negative control. In addition, protocols are designed to evaluate L-selectin-dependent homing of human lymphocytes to peripheral lymph nodes of SCID mice in vivo. This aim will be accomplished by examining the tissue localization of xenotransfused human lymphocytes that express high or low levels of the L-selectin peripheral lymph node homing receptor. The role of the L-selectin lymphocyte homing receptor in mediating human lymphocyte localization to peripheral lymph nodes will be further determined by treating human lymphocytes with L- selectin-specific blocking antibodies prior to xenotransfusion into SCID mice. Successful development of an in vivo model to study human homing will represent a major advance to multiple disciplines. This model will provide a unique system in which to explore the role of L-selectin as well as other adhesion pathways in normal lymphocyte homing and in pathological processes such as inflammation and metastasis. Moreover, this model should prove useful for evaluating new therapeutic strategies in these pathological disorders.